Preformed Pulse-Guiding Channels in Plasmas

The propagation of a laser pulse focused in a plasma at high intensity is affected by several linear and nonlinear phenomena, which can modify its amplitude and therefore its possibility to travel in the medium maintaining its original interaction conditions. The main limitation arises from optical diffraction that enables the propagation at the maximum intensity only over the Rayleigh length Z i. 

Optical guiding in preformed plasmas has been extensively investigated in experiments mainly oriented to demonstrate the production of relativistic electrons in LWF-related schemes. Plasma channel formation has been pursued with a variety of means, ranging from the use of hydrodynamic and shock-wave 

An alternative way to preform a channeled plasma consists in exploiting the nanosecond precursor that usually precedes a short femtosecond pulse in the output of a multi-terawatt laser system. In fact, the amplified spontaneous emission (ASE) pedestal has typically an intensity 106-1010 times lower than the main pulse, which, however, can be sufficient to ionize a gas-jet or a solid target. This drawback can be turned into a benefit assuming that this long precursor can prepare the plasma channel for the short pulse propagation. 

Finally, it is worth mentioning that with the advent of ultra-high intensity laser systems, the possibility for an ultrashort laser pulse to be self-guided via 

---